Add a typed `col` function for creating column references

[iravid]

Resolves #186.

Would be a good idea to wait for #110 before merging this to avoid conflict on SelectTests.scala.

[iravid]

Also refs #164

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@048d06c). Click here to learn what that means. The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #187   +/-   ##
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  Coverage          ?   94.95%           
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  Files             ?       35           
  Lines             ?      674           
  Branches          ?       11           
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  Hits              ?      640           
  Misses            ?       34           
  Partials          ?        0

Impacted Files	Coverage Δ
...ss/functions/PartiallyConstructedTypedColumn.scala	100% <100%> (ø)
...t/src/main/scala/frameless/functions/package.scala	100% <100%> (ø)

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[iravid]

@OlivierBlanvillain have another look; this approach should allow to keep the column reference in a value, for example, and then have the actual TypedColumn constructed when using select.

[iravid]

Ah, but this approach doesn't support expressions like val c = functions.col('a) + 1. Tradeoffs, tradeoffs...

[iravid]

What's the reasoning behind carrying around the dataset type in the typed columns?

[OlivierBlanvillain]

If it was just expr: TypedColumn[Boolean] we could't statically check that ds.select(expr) does not uses some columns that are not defined on ds. #162 has an interesting discussion with an alternative designed based around an implicit that would be the perfect solution if we had implicit function types.

[iravid]

That's true, but how about moving the column existence evidence to the methods on TypedDataset?

The reasoning is that a TypedColumn can freely exist outside the context of a typed dataset, e.g. as a literal expression. The evidence is only needed when using it as a projection on a dataset.

[OlivierBlanvillain]

We didn't consider that, but it sounds isomorphic given than we can already do something like def a(ds)(implicits) = ds('a)... Also I'm not sure it would work out with the requirements of #175.

[iravid]

It is isomorphic up to the requirement of having ds at hand ;)

I will open a separate PR to experiment with moving the evidence to ds.select et al and we can decide separately if this PR is good enough to be useful given its limited inference.